968 research outputs found
Patterns of habitat use and activity in British populations of sika deer of contrasting environments
Head-Tail Clouds: Drops to Probe the Diffuse Galactic Halo
A head-tail high-velocity cloud (HVC) is a neutral hydrogen halo cloud that
appears to be interacting with the diffuse halo medium as evident by its
compressed head trailed by a relatively diffuse tail. This paper presents a
sample of 116 head-tail HVCs across the southern sky (d < 2 deg) from the HI
Parkes All Sky Survey (HIPASS) HVC catalog, which has a spatial resolution of
15.5 arcmin (45 pc at 10 kpc) and a sensitivity of N_HI=2 x 10^(18) cm^(-2) (5
sigma). 35% of the HIPASS compact and semi-compact HVCs (CHVCs and :HVCs) can
be classified as head-tail clouds from their morphology. The clouds have
typical masses of 730 M_sun at 10 kpc (26,000 M_sun at 60 kpc) and the majority
can be associated with larger HVC complexes given their spatial and kinematic
proximity. This proximity, together with their similar properties to CHVCs and
:HVCs without head-tail structure, indicate the head-tail clouds have short
lifetimes, consistent with simulation predictions. Approximately half of the
head-tail clouds can be associated with the Magellanic System, with the
majority in the region of the Leading Arm with position angles pointing in the
general direction of the movement of the Magellanic System. The abundance in
the Leading Arm region is consistent with this feature being closer to the
Galactic disk than the Magellanic Stream and moving through a denser halo
medium. The head-tail clouds will feed the multi-phase halo medium rather than
the Galactic disk directly and provide additional evidence for a diffuse
Galactic halo medium extending to at least the distance of the Magellanic
Clouds.Comment: MNRAS Accepted, 10 figures, 7 in colo
Are Compact High-Velocity Clouds Extragalactic Objects?
Compact high-velocity clouds (CHVCs) are the most distant of the HVCs in the
Local Group model and would have HI volume densities of order 0.0003/cm^3.
Clouds with these volume densities and the observed neutral hydrogen column
densities will be largely ionized, even if exposed only to the extragalactic
ionizing radiation field. Here we examine the implications of this process for
models of CHVCs. We have modeled the ionization structure of spherical clouds
(with and without dark matter halos) for a large range of densities and sizes,
appropriate to CHVCs over the range of suggested distances, exposed to the
extragalactic ionizing photon flux. Constant-density cloud models in which the
CHVCs are at Local Group distances have total (ionized plus neutral) gas masses
roughly 20-30 times larger than the neutral gas masses, implying that the gas
mass alone of the observed population of CHVCs is about 40 billion solar
masses. With a realistic (10:1) dark matter to gas mass ratio, the total mass
in such CHVCs is a significant fraction of the dynamical mass of the Local
Group, and their line widths would exceed the observed FWHM. Models with dark
matter halos fare even more poorly; they must lie within approximately 200 kpc
of the Galaxy. We show that exponential neutral hydrogen column density
profiles are a natural consequence of an external source of ionizing photons,
and argue that these profiles cannot be used to derive model-independent
distances to the CHVCs. These results argue strongly that the CHVCs are not
cosmological objects, and are instead associated with the Galactic halo.Comment: 30 pages, 14 figures; to appear in The Astrophysical Journa
A Limit on the Metallicity of Compact High Velocity Clouds
There is a fortuitous coincidence in the positions of the quasar TonS210 and
the compact H I high velocity cloud CHVC224.0-83.4-197 on the sky. Using Far
Ultraviolet Spectroscopic Explorer observations of the metal-line absorption in
this cloud and sensitive H I 21cm emission observations obtained with the
multibeam system at Parkes Observatory, we determine a metallicity of (O/H)
<0.46 solar at a confidence of 3 sigma. The metallicity of the high velocity
gas is consistent with either an extragalactic or Magellanic Cloud origin, but
is not consistent with a location inside the Milky Way unless the chemical
history of the gas is considerably different from that of the interstellar
medium in the Galactic disk and halo. Combined with measurements of highly
ionized species (C III and O VI) at high velocities, this metallicity limit
indicates that the cloud has a substantial halo of ionized gas; there is as
much ionized gas as neutral gas directly along the Ton S210 sight line. We
suggest several observational tests that would improve the metallicity
determination substantially and help to distinguish between possible origins
for the high velocity gas. Additional observations of this sight line would be
valuable since the number of compact HVCs positioned in front of background
sources bright enough for high resolution absorption-line studies is extremely
limited.Comment: 15 pages, 3 postscript figures + 1 JPEG figure (reduced from
postscript for size considerations), accepted for publication in ApJ (June
2002
The Relationship Between Baryons and Dark Matter in Extended Galaxy Halos
The relationship between gas-rich galaxies and Ly-alpha absorbers is
addressed in this paper in the context of the baryonic content of galaxy halos.
Deep Arecibo HI observations are presented of two gas-rich spiral galaxies
within 125 kpc projected distance of a Ly-alpha absorber at a similar velocity.
The galaxies investigated are close to edge-on and the absorbers lie almost
along their major axes, allowing for a comparison of the Ly-alpha absorber
velocities with galactic rotation. This comparison is used to examine whether
the absorbers are diffuse gas rotating with the galaxies' halos, outflow
material from the galaxies, or intergalactic gas in the low redshift cosmic
web. The results indicate that if the gas resides in the galaxies' halos it is
not rotating with the system and possibly counter-rotating. In addition, simple
geometry indicates the gas was not ejected from the galaxies and there are no
gas-rich satellites detected down to 3.6 - 7.5 x 10^6 Msun, or remnants of
satellites to 5-6 x 10^{18} cm^{-2}. The gas could potentially be infalling
from large radii, but the velocities and distances are rather high compared to
the high velocity clouds around the Milky Way. The most likely explanation is
the galaxies and absorbers are not directly associated, despite the vicinity of
the spiral galaxies to the absorbers (58-77 kpc from the HI edge). The spiral
galaxies reside in a filament of intergalactic gas, and the gas detected by the
absorber has not yet come into equilibrium with the galaxy. These results also
indicate that the massive, extended dark matter halos of spiral galaxies do not
commonly have an associated diffuse baryonic component at large radii.Comment: Accepted by AJ, 33 pages preprint format, see
http://www.astro.lsa.umich.edu/~mputman/putman1.pdf for a higher resolution
versio
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